> -----Original Message-----
> From: Olin Lathrop [mailto:olin_piclist@EMBEDINC.COM]
>
> The PIC can help here but only turning on the thermistor long
> enough for
> the A/D voltage to settle, then off again next reading.  Since
> temperatures don't change that fast, you could easily turn on the
> thermistor for 100uS every 100mS.  The average current will
> then be 1/1000
> of the current used during a reading.

That's a good plan, and one I have used.  You do have to be carefull with
timing if the thermistor is decoupled to reduce noise.  Also some ceramic
capacitors often used for decoupling (notably X7R or Z5U material) have a
very poor temperature coefficient, so the timing must make allowances for
this.

Using the Microchip AN512 style method of measuring resistance via a bit
bashed single slope ADC is also surprisingly accurate, especialy if the PIC
has a comparator input.

Mike




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